Gears are in frequent use in transmissions for automobiles and so on. In recent years, an improvement in the machining accuracy of the gears has been desired with the aim of reducing vibrations and noise of the transmissions. With the gear machining method, it is common practice to carry out gear cutting in a predetermined gear material, thereby forming a tooth profile, heat-treat the gear formed by gear cutting, and then perform finishing (grinding) in order to remove strain or the like due to this heat treatment. So far, the external gear to be machined after heat treatment and a grindstone have been put in mesh, with an angle of axis intersection being given, and the tooth flanks of the external gear to be machined have been ground. Tools used for these grinding processes include tools available in various shapes, such as an external gear shape, an internal gear shape, and a screw (worm) shape, in accordance with the shape of the gear to be ground.
As grinding proceeds, the grindstone undergoes loading or wear, and its sharpness decreases. After the grindstone has ground a predetermined number of gears, therefore, it becomes necessary to perform dressing or truing on the grindstone having a worn-out grinding surface, thereby regenerating a sharp cutting edge surface.
As a method for carrying out dressing or truing, it is performed to dress the grindstone with the use of a dressing gear set to have nearly the same gear specifications as those of the finished gear. Patent Document 1 relates to a hard gear honing process using an internally toothed grindstone on a gear after heat treatment, and discloses a dressing gear whose number of teeth is set to be equal to or more than 1.5 times the number of teeth of the gear to be machined (work) in order to perform high accuracy dressing or truing of the internally toothed grindstone.